Non-Coplanar Diphenyl Fluorene and Weakly Polarized Cyclohexyl Can Effectively Improve the Solubility and Reduce the Dielectric Constant of Poly (Aryl Ether Ketone) Resin

With the rapid development of high-frequency communication and large-scale integrated circuits, insulating dielectric materials require a low dielectric constant and dielectric loss. Poly (aryl ether ketone) resins (PAEK) have garnered considerable attention as an intriguing class of engineering thermoplastics possessing excellent chemical and thermal properties. However, the high permittivity of PAEK becomes an obstacle to its application in the field of high-frequency communication and large-scale integrated circuits. Therefore, reducing the dielectric constant and dielectric loss of PAEK while maintaining its excellent performance is critical to expanding the PAEK applications mentioned above. This study synthesized a series of poly (aryl ether ketone) resins that are low dielectric, highly thermally resistant, and soluble, containing cyclohexyl and diphenyl fluorene. The effects of cyclohexyl contents on the properties of a PAEK resin were studied systematically. The results showed that weakly-polarized cyclohexyl could reduce the molecular polarization of PAEK, resulting in low permittivity and high transmittance. The permittivity of PAEK is 2.95–3.26@10GHz, and the transmittance is 65–85%. In addition, the resin has excellent solubility and can be dissolved in NMP, DMF, DMAc, and other solvents at room temperature. Furthermore, cyclohexyl provided PAEK with excellent thermal properties, including a glass transition temperature of 239–245 °C and a 5% thermogravimetric temperature, under a nitrogen atmosphere of 469–534 °C. This makes it a promising candidate for use in high-frequency communications and large-scale integrated circuits.


Introduction
The rapid development of high-frequency communication and large-scale integrated circuits has raised new requirements for interlayer insulating dielectric materials [1][2][3]. Insulating dielectric materials are required to have a low dielectric constant and low dielectric loss under high frequency, as well as excellent thermal and mechanical properties, processability, dimensional stability, and low water absorption [4][5][6].
Traditional general polymeric materials, such as polyethylene (PE), polypropylene (PP), etc., have low permittivity and dielectric loss but fail to meet requirements due to application-temperature mismatch. Numerous special engineering plastics, such as traditional polyimide (PI) and liquid crystal polymer (LCP), were expected to meet requirements due to their superior thermal and mechanical performance and dielectric properties [7,8]. However, the dielectric constant (D k ) of 3.0-3.5 is insufficient for application in high-frequency communication and large-scale integrated circuits. Based on the Clausius-Mossotti equation, low-dielectric materials can be prepared by reducing thermal properties, and solubility properties. It was shown that the introduction of weaklypolarized cyclohexyl groups into the backbone of PAEK effectively improves the solubility, optical transmittance, and dielectric constant of the resin while maintaining its excellent thermal resistance and mechanical properties. This makes it a promising candidate for use in high-frequency communications and large-scale integrated circuits.

Synthesis of DFBCH and BFBB
The synthetic route of DFBCH and BFBB is shown in Scheme 1. A total of 140 g (0.813 mol) CHDA, 280 mL thionyl chloride, and a few drops of DMF were added to a 1 L three-necked flask equipped with a magnetic stirring under nitrogen flow, condenser tube and gas treatment device. The reaction was carried out at 80 • C until the system was transparent, and the excess thionyl chloride was removed by distillation under reduced pressure. The above-synthesized material and 210 g (2.1851 mol) fluorobenzene were added to a 1 L three-necked flask equipped with mechanical stirring under nitrogen flow and spherical condenser ice bath conditions. After the CHDC was fully dissolved, anhydrous aluminum trichloride (214 g, 1.6050 mol) was added, increasing the temperature to 70 • C and maintaining for 12 h. It was then cooled to room temperature and poured into an aqueous solution containing crushed ice, methanol, and hydrochloric acid. The coarse DFBCH was obtained by filtration after vigorous stirring. The white crystalline DFBCH was obtained by double recrystallization with DMF, yield 51% (Supplementary Materials: 1H-NMR, Figure S1; FT-IR spectrum, Figure S2; MS spectrum, Figure S3). The synthesis of BFBB is referred to as the DFBCH synthesis method. The pure BFBB monomer was obtained after two recrystallizations with DMAc, yield 60% (Supplementary Materials: 1H-NMR, Figure S4; FT-IR spectrum, Figure S5; MS spectrum, Figure S6). prepared by solution polycondensation of DFBCH, 9,9-Bis(4-hydroxyphenyl) fluorene (BHPF), and 1,4-bis(4-fluorobenzoyl) benzene (BFBB). The material's properties were tuned by controlling the content ratio of the two groups. The effects of the cyclohexyl contents on the properties of the resin were systematically investigated, especially dielectric properties, thermal properties, and solubility properties. It was shown that the introduction of weakly-polarized cyclohexyl groups into the backbone of PAEK effectively improves the solubility, optical transmittance, and dielectric constant of the resin while maintaining its excellent thermal resistance and mechanical properties. This makes it a promising candidate for use in high-frequency communications and large-scale integrated circuits.

Synthesis of DFBCH and BFBB
The synthetic route of DFBCH and BFBB is shown in Scheme 1. A total of 140 g (0.813 mol) CHDA, 280 mL thionyl chloride, and a few drops of DMF were added to a 1 L threenecked flask equipped with a magnetic stirring under nitrogen flow, condenser tube and gas treatment device. The reaction was carried out at 80 °C until the system was transparent, and the excess thionyl chloride was removed by distillation under reduced pressure. The above-synthesized material and 210 g (2.1851 mol) fluorobenzene were added to a 1 L three-necked flask equipped with mechanical stirring under nitrogen flow and spherical condenser ice bath conditions. After the CHDC was fully dissolved, anhydrous aluminum trichloride (214 g, 1.6050 mol) was added, increasing the temperature to 70 °C and maintaining for 12 h. It was then cooled to room temperature and poured into an aqueous solution containing crushed ice, methanol, and hydrochloric acid. The coarse DFBCH was obtained by filtration after vigorous stirring. The white crystalline DFBCH was obtained by double recrystallization with DMF, yield 51% (Supplementary Materials: 1H-NMR, Figure S1; FT-IR spectrum, Figure S2; MS spectrum, Figure S3). The synthesis of BFBB is referred to as the DFBCH synthesis method. The pure BFBB monomer was obtained after two recrystallizations with DMAc, yield 60% (Supplementary Materials: 1H-NMR, Figure  S4; FT-IR spectrum, Figure S5; MS spectrum, Figure S6).

Synthesis of PCBEKs
Using BHPF, BFBB, and DFBCH as raw materials, poly (aryl ether ketone) resins (PCBEKs) containing cyclohexyl/fluorene structures were synthesized by solution polycondensation with different monomer ratios. The synthetic route is shown in Scheme 2. Resins were named according to the percentage of DFBCH and BFBB. For example, the copolymer resin is named PCBEK-C75B25 when the ratio of DFBCH to BFBB monomeric feedstock is 75:25. In addition, to exclude the influence of the molecular weight of the resin on its macroscopic properties, the molecular weight of the resin was uniformly designed to be 30,000. The preparation of PCBEK-C75B25 is presented below as an example. First, added the BHPF (5.5506 g, 15.84 mmol), DFBCH (4.0093 g, 12.21 mmol), BFBB (1.2764 g, 3.96 mmol), anhydrous potassium carbonate (3.0648 g, 22.176 mmol), 10 mL of mixed solvent (7/1, sulfone/NMP), and 20 mL of toluene into a 100 mL three-necked flask equipped with nitrogen flow, mechanical stirring, a water separation device, and a condensation reflux tube in turn. The reaction system is then heated to a reflux state to promote the BHPF production of phenoxide, and the by-product, water, is brought out with the reflux of toluene. After the water is entirely carried out, the toluene is distilled out and the reaction system is gradually heated to 190 • C. The solvent is added appropriately during the reaction period to suppress locally reactions that are too fast. The reaction is stopped when the viscosity of the reaction system no longer increases. The mixture is then poured into hot water containing hydrochloric acid, and a white fibrous product polymer is obtained, which is the crude product of PCBEK-C75B25. The synthetic route of DFBCH and BFBB is shown in Scheme 1.140g (0.813mol) 85 CHDA, 280ml thionyl chloride, and a few drops of DMF were added to a 1 L three-necked 86 flask equipped with magnetic stirring and nitrogen flow, condenser tube and gas treat-87 ment device. The reaction was carried out at 80 ºC until the system was transparent, and 88 the excess thionyl chloride was removed by distillation under reduced pressure. The 89 above-synthesized material and 210g (2.1851mol) fluorobenzene were added to a 1L three-90 necked flask equipped with mechanical stirring under nitrogen flow and spherical con-91 denser ice bath conditions. After the CHDC was fully dissolved, add anhydrous alumi-92 num trichloride (214g, 1.6050mol), increasing the temperature to 70°C and maintaining 93 for 12 hours. It is then cooled to room temperature and poured into an aqueous solution 94 containing crushed ice, methanol, and hydrochloric acid. The coarse DFBCH is obtained 95 by filtration after vigorous stirring. The white crystalline DFBCH is obtained by double 96 recrystallization with DMF. Yield, 51%. (1H-NMR, Figure S1, FT-IR spectrum, Figure S2, 97 MS spectrum, Figure S3, Supporting Information). The synthesis of BFBB is referred to as 98 the DFBCH synthesis method. The pure BFBB monomer is obtained by two recrystalliza-99 tions with DMAc. Yield, 60%. (1H-NMR, Figure S4, FT-IR spectrum, Figure S5, MS spec-100 trum, Figure S6, Supporting Information). 101   Using bisphenol fluorene (BHPF), BFBB, and DFBCH as raw materials, poly (aryl 105 ether ketone) resin (PCBEKs) containing cyclohexyl/fluorene structures were synthesized 106 by solution polycondensation with different monomer ratios. The synthetic route is 107 shown in Scheme 2. Resins are named according to the percentage of DFBCH and BFBB. 108 For example, the copolymer resin is named PCBEK-C75B25 when the ratio of DFBCH to 109 BFBB monomeric feedstock is 75:25. In addition, to exclude the influence of the molecular 110 weight of the resin on its macroscopic properties, the molecular weight of the resin was 111 uniformly designed to be 30, 000. The preparation of PCBEK-C75B25 is presented below 112 as an example. First, in a 100 mL three-necked flask equipped with nitrogen flow, me-113 chanical stirring, water separation device, and condensation reflux tube, BHPF (5.5506g, 114 15.84mmol), DFBCH (4.0093g, 12.21mmol), BFBB (1.2764g, 3.96mmol), and anhydrous po-115 tassium carbonate (3.0648g, 22.176mmol), 10 mL of mixed solvent (7/1, sulfone/NMP), and 116 20 mL of toluene. The reaction system is then heated to a reflux state to promote the BHPF 117 Scheme 2. Synthesis of PCBEKs.

Preparation of PCBEKs Films
First, the copolymer (1.5 g) was accurately weighed, the solvent NMP (10.5 g) was added, and the ultrasonic treatment was followed by 1 h of agitation at room temperature until the polymer was completely dissolved. The solution was uniformly coated onto a clean, dry glass plate. Last, the glass plate covered with the solution was treated with a series of temperatures (60 • C, 2 h; 800 • C, 1 h; 120 • C, 2 h; 160 • C, 1 h; 200 • C, 2 h) to obtain a PCBEKs copolymer film.

Characterization
The FT-IR spectra of the resins were recorded at room temperature using a Shimadzu IR Affinity-1 spectrophotometer in the 400 to 4000 cm −1 range. Bruker AVANCE III carried out NMR measurements, and the solvent was deuterated chloroform (CDCl3) or deuterated dimethyl sulfoxide (DMSO-d6). Molecular weight information of the resins was obtained on Agilent PL 200. The thermal properties of the resin were analyzed using TA-Q50 and Netzsch DSC-200F3. The samples were dried at 120 • C for 24 h in a vacuum oven before the test. Dielectric property tests were executed via a vector network analyzer P5004A-200 (Keysight) at 10 GHz room temperature. Solubility tests were performed at room temperature at 0.05 g/mL in different solvents. Wide-angle X-ray diffraction (WAXD) experiments on polymer films were obtained on Rigaku MiniFlex600 using a Cu-Kα radiation (45 kV, 15 mA) source in the range of 5-80 • with a scanning rate of 10 • /min. The tensile strength of the resin was determined using a SUST CMT5000, according to the ASTM D882-2018 standard. The optical transmittance of the PCBEKs was measured by a UV-3600 Plus (Shimadzu, Japan) at wavelengths from 300 to 800 nm.

Structural Characterization
PCBEKs resin films were characterized using ATR-FTIR to determine the functional groups in the molecular chains of the copolymers. As shown in Figure 1, the five characteristic absorption peaks correspond to four distinct functional groups of the copolymer molecular chain. The absorption peak at 1230 cm −1 wavenumber is assigned to the C-O-C stretching vibrational of aromatic ether. This indicates that the nucleophilic substitution reaction was successfully performed and formed ether bonds. The absorption peak at 1654 cm −1 wavenumber is assigned to the C=O stretching vibrational peak of aromatic ketones. The absorption peaks at 1587 cm −1 and 1490 cm −1 are characteristic absorption peaks of the benzene ring skeleton. The absorption peak at 2927 cm −1 wavenumber is assigned to the stretched vibrational absorption peak of -CH 2 -on the cyclohexane skeleton. Infrared test results preliminarily confirmed that the poly (aryl ether ketone) resin PCBEKs containing cyclohexyl/fluorene structure were successfully prepared.
The FT-IR spectra of the resins were recorded at room temperature using a Shimadzu IR Affinity-1 spectrophotometer in the 400 to 4000 cm −1 range. Bruker AVANCE Ⅲ carried out NMR measurements, and the solvent was deuterated chloroform (CDCl3) or deuterated dimethyl sulfoxide (DMSO-d6). Molecular weight information of the resins was obtained on Agilent PL 200. The thermal properties of the resin were analyzed using TA-Q50 and Netzsch DSC-200F3. The samples were dried at 120 °C for 24 h in a vacuum oven before the test. Dielectric property tests were executed via a vector network analyzer P5004A-200 (Keysight) at 10 GHz room temperature. Solubility tests were performed at room temperature at 0.05 g/mL in different solvents. Wide-angle X-ray diffraction (WAXD) experiments on polymer films were obtained on Rigaku MiniFlex600 using a Cu-Kα radiation (45 kV, 15 mA) source in the range of 5-80° with a scanning rate of 10°/min. The tensile strength of the resin was determined using a SUST CMT5000, according to the ASTM D882-2018 standard. The optical transmittance of the PCBEKs was measured by a UV-3600 Plus (Shimadzu, Japan) at wavelengths from 300 to 800 nm.

Structural Characterization
PCBEKs resin films were characterized using ATR-FTIR to determine the functional groups in the molecular chains of the copolymers. As shown in Figure 1, the five characteristic absorption peaks correspond to four distinct functional groups of the copolymer molecular chain. The absorption peak at 1230 cm −1 wavenumber is assigned to the C-O-C stretching vibrational of aromatic ether. This indicates that the nucleophilic substitution reaction was successfully performed and formed ether bonds. The absorption peak at 1654 cm −1 wavenumber is assigned to the C=O stretching vibrational peak of aromatic ketones. The absorption peaks at 1587 cm −1 and 1490 cm −1 are characteristic absorption peaks of the benzene ring skeleton. The absorption peak at 2927 cm −1 wavenumber is assigned to the stretched vibrational absorption peak of -CH2-on the cyclohexane skeleton. Infrared test results preliminarily confirmed that the poly (aryl ether ketone) resin PCBEKs containing cyclohexyl/fluorene structure were successfully prepared. The structure of the copolymer was further analyzed by 1H-NMR, and the results are shown in Figure 2. The signal peaks with δ = 3.98 ppm, δ = 1.94 ppm, and δ = 1.57 ppm are assigned to H2 and H3 of the cyclohexyl structure on DFBCH, respectively. The signal The structure of the copolymer was further analyzed by 1H-NMR, and the results are shown in Figure 2. The signal peaks with δ = 3.98 ppm, δ = 1.94 ppm, and δ = 1.57 ppm are assigned to H2 and H3 of the cyclohexyl structure on DFBCH, respectively. The signal peaks at 7.82 ppm and δ = 7.73 ppm are assigned to H1 and H11. The signal regions H1, H2, and H3 increase with cyclohexanedione content. Based on this, H1 and H11 were selected as the two characteristic protons for comparison. The results showed that the theoretical ratios (1:0, 3:2, 1:2, 2:3, and 0:2) of H1 and H11 were near to the actual ratios (1:0, 3:1.95, 1:1.97, 2:2.08, and 0:2); it can be argued that the actual content ratios of DFBCH and BFBB in repeating units are consistent with the theoretical content ratios. In summary, it can be inferred that the polymer structure is compatible with the design. reaction is successfully performed, and ether bonds are formed. The absorption pea 1654 cm -1 wavenumber is assigned to the C=O stretching vibrational peak of aromatic tones. The absorption peaks at 1587 cm-1 and 1490 cm -1 are characteristic absorption pe of the benzene ring skeleton. The absorption peak at 2927 cm -1 wavenumber is assig to the stretched vibrational absorption peak of -CH2on the cyclohexane skeleton. Infra test results preliminarily confirmed that the poly (aryl ether ketone)s resin PCBEKs taining cyclohexyl/fluorene structure were successfully prepared. The structure of the copolymer was further analyzed by 1H-NMR and the results shown in Fig.2. The signal peaks with δ=3.98 ppm, δ=1.94 ppm and δ=1.57 ppm are signed to H2 and H3 of the cyclohexyl structure on DFBCH, respectively. The signal pe at 7.82 ppm and δ=7.73 ppm should be H1 and H11. The signal regions H1, H2, and increase with cyclohexanedione content. Based on this, H1 and H11 were selected as two characteristic protons for comparison. The results showed that the theoretical ra (1:0, 3:2, 1:2, 2:3 and 0:2) of H1 and H11 were near to the actual ratios (1: 0, 3: 1.95, 1: 1 2: 2.08 and 0: 2); It can be argued that the actual content ratios of DFBCH and BFB repeating units are consistent with the theoretical content ratios. In summary, it can inferred that the polymer structure is compatible with the design. It can be seen from the GPC test results (

Thermal Properties
Differential scanning calorimeters (DSC) were used to characterize the thermal properties of the copolymer resin. The results are shown in Figure 3 and Table 2. According to the second heating curve of the PCBEK series of resins, only one glass transition process, and no melting process, appears. This indicates that the copolymers were an amorphous structure that matched the XRD results ( Figure S7, Supplementary Materials). The glass transition temperature (Tg) of resins is between 239.1 • C and 245.4 • C, increasing with the cyclohexanedione content. This is about 100 • C higher than commercial PEEK resin. Presumably, the anti-conformation of the cyclohexanedione monomer improves the molecular chain, which constrains the motion of the polymer at higher glass transition temperatures. transition temperature (Tg) of resins is between 239.1 °C and 245.4 °C, increasing with the cyclohexanedione content. This is about 100 °C higher than commercial PEEK resin. Presumably, the anti-conformation of the cyclohexanedione monomer improves the molecular chain, which constrains the motion of the polymer at higher glass transition temperatures. In addition, based on the relationship between the Tg of different monomers and their feed ratio, the Tg value of the copolymer can be predicted by the Fox equation, shown in Formula (1): (1) where Tg is the glass transition temperature of the copolymer, W1, and W2 are the mass fractions of a single bisphenol monomer in the total bisphenol, respectively, and Tg1 and Tg2 are the glass transition temperatures of the corresponding homopolymers. The Tg value of the copolymer measured by DSC and calculated by the Fox equation is shown in Figure 3b. The agreement between experimental and theoretical values of Tg indicates that the structure of copolymers with varying monomer ratios is consistent with predictions.
The thermal stability of the resin was characterized using a thermogravimetric analyzer (TGA), and the results are shown in Figure 4. The PCBEK resins have excellent thermal stability and can exist stably at 450 °C. In the range of 469-534 °C, the resins' weight can maintain more than 95%. It can be seen from Tables 1 and 2 that, with the increase of phenyl content, the thermal stability of the resin is increasingly excellent; the Td5% of PCBEK-C0B100 is 534 °C, and the Td10% is 551 °C. The carbon residue rate (Cy) at 800 °C is more than 75%. However, the trend of thermal stability of the resins is opposite to the direction of the glass transition temperature trend. The reason is presumed to be that the stability of the alicyclic ring is weaker than that of the benzene ring. The benzene ring structure is steeper and, therefore, more difficult to open than the alkyl rings.   In addition, based on the relationship between the T g of different monomers and their feed ratio, the T g value of the copolymer can be predicted by the Fox equation, shown in Formula (1): where T g is the glass transition temperature of the copolymer, W 1 , and W 2 are the mass fractions of a single bisphenol monomer in the total bisphenol, respectively, and T g1 and T g2 are the glass transition temperatures of the corresponding homopolymers. The T g value of the copolymer measured by DSC and calculated by the Fox equation is shown in Figure 3b. The agreement between experimental and theoretical values of T g indicates that the structure of copolymers with varying monomer ratios is consistent with predictions. The thermal stability of the resin was characterized using a thermogravimetric analyzer (TGA), and the results are shown in Figure 4. The PCBEK resins have excellent thermal stability and can exist stably at 450 • C. In the range of 469-534 • C, the resins' weight can maintain more than 95%. It can be seen from Tables 1 and 2 that, with the increase of phenyl content, the thermal stability of the resin is increasingly excellent; the T d5% of PCBEK-C0B100 is 534 • C, and the T d10% is 551 • C. The carbon residue rate (Cy) at 800 • C is more than 75%. However, the trend of thermal stability of the resins is opposite to the direction of the glass transition temperature trend. The reason is presumed to be that the stability of the alicyclic ring is weaker than that of the benzene ring. The benzene ring structure is steeper and, therefore, more difficult to open than the alkyl rings.
In summary, the thermal performance test results show that the poly (aryl ether ketone) resin containing the cyclohexyl/diphenylfluorene group generally exhibits excellent thermal resistance and thermal stability, and the thermal properties can be adjusted to meet different practical application needs.   In summary, the thermal performance test results show that the poly (a tone) resin containing the cyclohexyl/diphenylfluorene group generally exhi thermal resistance and thermal stability, and the thermal properties can be meet different practical application needs.

Mechanical Properties
The resin was made into a film and cut to produce a sample with a part The tensile stress-strain properties of all samples were measured at room The tensile strength, Young's modulus, and elongation of the copolymer at t ues are shown in Figure 5, and the relevant parameters are presented in Tab results show that the poly (aryl ether ketone) resin containing the cyclohexyl/ orene structure has excellent mechanical properties. First, the tensile str PCBEK resin is between 57.2 MPa and 64.5 Mpa, decreasing as cyclohexy creases. This may be because the weakly-polarized cyclohexyl group destro gation effect within the molecular chain, and thus weakens the inter-and int interactions. Furthermore, Young's modulus of the copolymer decreases from

Mechanical Properties
The resin was made into a film and cut to produce a sample with a particular shape. The tensile stress-strain properties of all samples were measured at room temperature. The tensile strength, Young's modulus, and elongation of the copolymer at the break values are shown in Figure 5, and the relevant parameters are presented in Table 3. The test results show that the poly (aryl ether ketone) resin containing the cyclohexyl/diphenylfluorene structure has excellent mechanical properties. First, the tensile strength of the PCBEK resin is between 57.2 MPa and 64.5 Mpa, decreasing as cyclohexyl content increases. This may be because the weakly-polarized cyclohexyl group destroys the conjugation effect within the molecular chain, and thus weakens the inter-and intra-molecular interactions. Furthermore, Young's modulus of the copolymer decreases from 1.33 GPa to 1.05 GPa as the cyclohexyl group in the backbone increases. This may be due to the weaker rigidity of cyclohexyl compared to phenyl. The flexibility of cyclohexyl endows the molecular chains with increased elongation and toughness. In addition, the elongation at the break of the polymer, from 7.45% to 15.71%, increases with the cyclohexyl content, which additionally supports the above analysis. Although the mechanical properties of the modified PAEK resin are reduced, it still meets the requirements of high-frequency communication and large-scale integrated circuits.

Optical Transparency Properties
The optical properties of the film were examined with a UV-Vis-NIR spectrometer. The results of the tests are shown in Figure 6 and Table 4. The cutoff wavelengths of polymer films are in the range of 340 nm to 385 nm. Among these, PCBEK-C100B0 has a cut-off wavelength of 340 nm, and PCBEK-C0B100 has a cut-off wavelength of 385 nm. In addition, polymer films have excellent optical transmittance in the visible light spectrum. The transmittance of PCBEK-C100B0 at 450 nm is 85.81%, which is the best in the series. This is because cyclohexyl units disrupt the conjugation of the molecular chain, which may weaken the effect of electron transfer on the molecular chain [25]. As mentioned above, the resultant copolymers can be widely used as a new optical material in the application of optical thin films.

Optical Transparency Properties
The optical properties of the film were examined with a UV-Vis-NIR spectrometer. The results of the tests are shown in Figure 6 and Table 4. The cutoff wavelengths of polymer films are in the range of 340 nm to 385 nm. Among these, PCBEK-C100B0 has a cutoff wavelength of 340 nm, and PCBEK-C0B100 has a cut-off wavelength of 385 nm. In addition, polymer films have excellent optical transmittance in the visible light spectrum. The transmittance of PCBEK-C100B0 at 450 nm is 85.81%, which is the best in the series. This is because cyclohexyl units disrupt the conjugation of the molecular chain, which may weaken the effect of electron transfer on the molecular chain [25]. As mentioned above, the resultant copolymers can be widely used as a new optical material in the application of optical thin films.

Solubility Properties
The solubility test results (Table 5) show that PCBEKs can be dissolved in aprotic polar solvents such as NMP, chloroform, DMAc, and DMF at room temperature. In addition, they were partially dissolved in DMSO and sulfolane at room temperature, while soluble after heating. The reason for this is that the kinked and non-coplanar structure of diphenylfluorene breaks the regular arrangement of the molecular chains, enlarging the gaps between the chains and making it easier for the solvent to penetrate, increasing solubility. This excellent solubility makes the copolymer easy to prepare as a thin film by solution pouring.

Dielectric Properties
This study aimed to design a poly (aryl ether ketone) resin with high thermal resistance and low permittivity. The dielectric properties of the resin were characterized using a vector network analyzer, and the results are shown in Table 6. The obtained films show excellent dielectric performance, and the dielectric constant (D k ) and dielectric loss (D f ) are 2.95-3.26@10GHz and 0.033-0.041@10GHz, respectively. The dielectric constant of the resin decreases with increasing cyclohexyl content. PCBEK-C100B0 has the best dielectric properties, with a D k of 2.95 and a D f of 0.036 at 10 GHz. Presumably, diphenylfluorene disrupts the regularity of the molecular structure and increases the intermolecular volume. The weakly-polarized cyclohexyl group further reduces the molar polarizability of the molecule. Therefore, the dielectric properties of PCBEK-C100B0 are the best. In addition, low-dielectric materials should be kept as dry as possible during use, as water can significantly impair the permittivity and dielectric loss, and can even damage the device. Wang et al. showed that cyclohexyl could increase the hydrophobicity of resins, which may be another reason for the low dielectric constant of the obtained materials [18]. In conclusion, the introduction of cyclohexyl can effectively reduce the dielectric constant of resin, making it possible for PAEK resins to be used as a candidate for insulating dielectric materials.

Conclusions
This study synthesized a series of poly (aryl ether ketone) resins that are low dielectric, highly thermally resistant, and soluble, containing cyclohexyl and diphenyl fluorene. The effects of cyclohexyl contents on the properties of a PAEK resin were studied systematically. The results showed that weakly-polarized cyclohexyl groups could reduce the molecular polarization of PAEK, resulting in low permittivity and high transmittance of PAEK, making it possible for PAEK resins to be used as a candidate for insulating dielectric materials. Furthermore, aromatic and non-coplanar diphenyl fluorene can destroy the regularity of PAEK's molecular chain and provide PAEK with excellent solubility and enriched resin molding processing methods. Herein, it is considered a suitable method for adjusting diphenyl fluorene and cyclohexyl content to acquire low-dielectric insulating materials with comprehensive properties that meet the actual requirements.